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ustainability considerations for electricity generation from biomass

nnette Evans, Vladimir Strezov *, Tim J. Evans

aduate School of the Environment, Faculty of Science, Macquarie University, Sydney, NSW 2109, Australia

ntents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1419

2. Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

2.1. Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

2.2. Gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

2.3. Direct combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

3. Biomass types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

3.1. Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

3.1.1. Bagasse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

3.1.2. Forest and non-bagasse agricultural residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1421

3.2. Dedicated energy crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1421

4. Sustainability assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1421

4.1. Price. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422

4.1.1. Total cost of electricity production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422

4.1.2. Investment costs and technology choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422

4.1.3. Process versus feedstock price . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422

4.2. Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1423

4.3. Greenhouse gas emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1423

4.4. Water use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424

4.5. Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424

4.6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424

4.7. Land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424

4.8. Social impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425

Renewable and Sustainable Energy Reviews 14 (2010) 1419–1427

R T I C L E I N F O

ticle history:

ceived 15 December 2009

cepted 15 January 2010

ywords:

omass

ectricity

stainability

issions

ice

A B S T R A C T

The sustainability of electricity generation from biomass has been assessed in this work according to the

key indicators of price, efficiency, greenhouse gas emissions, availability, limitations, land use, water use

and social impacts. Biomass produced electricity generally provides favourable price, efficiency,

emissions, availability and limitations but often has unfavorably high land and water usage as well as

social impacts. The type and growing location of the biomass source are paramount to its sustainability.

Hardy crops grown on unused or marginal land and waste products are more sustainable than dedicated

energy crops grown on food producing land using high rates of fertilisers.

� 2010 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

journa l homepage: www.e lsev ier .com/ locate / rser

* Corresponding author. Tel.: +61 2 9850 6959; fax: +61 2 9850 7972.

E-mail address: vstrezov@gse.mq.edu.au (V. Strezov).

64-0321/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

i:10.1016/j.rser.2010.01.010

1. Introduction

The generation of electricity has become one of the keytechnological advancements linked to high quality of life in modernsociety. As global populations increase and third world nations

Fig. 1. Biomass and total world electricity generation time trend (Euromonitor

International, 2009) [1].

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–14271420

develop, electricity demand continues to grow. Energy security is amatter of global significance, energy sources need to be available inconstant supply and at low cost. Significant attention is also focusedon the environmental impacts of electricity generation as highpolluting technologies attract public outrage. As information comesto light about the damaging health and environmental effects of theconventional methods of electricity generation, alternateapproaches are being sought. With limited fossil fuel reserves andtheir volatile prices, renewable fuels can provide increased energysecurity and stable price profiles.

Biomass is organic, plant derived material that may be convertedinto other forms of energy. It is easily produced in almost anyenvironment, regenerated quickly and has a long history of use fordirect heating applications. Biomass is the only fuel available forrenewable, combustion based electricity generation. For thesereasons, it has gained significant attention as a substitute for fossilfuels.

The use of biomass to produce electricity has steadily increasedby an average of 13 TWh per year between 2000 and 2008. Biomassbased electricity has maintained its market share of total globalgeneration over the last 20 years, at approximately 2%. This isshown in Fig. 1 [1].

The use of biomass is widespread, as shown in Fig. 2 [1]. Thereare a total of 62 countries in the world currently producingelectricity from biomass [1]. The USA is the dominant biomasselectricity producer at 26% of world production, followed byGermany (15%), Brazil and Japan (both 7%).

Biomass could help to ensure global energy security (primaryconcern) and help to mitigate carbon pollution (secondary

Fig. 2. Global distribution of biomass energy use in 2008 (Euromonitor

International, 2009) [1].

concern) [2]. Maintaining a primary focus on energy securitycould have economic impacts, where some price premium isconsidered appropriate for such security.

To understand the potential of biomass to contribute moresignificantly to global electricity generation, a full assessment of itspotential for sustainable development should be conducted.Previous studies have included detailed investigations on theoptimisation of crop growth and processing [3,4], variability ofbiomass availability and costs between countries [5] and theperformance of specific types of biomass between differenttechnologies [6]. A large number of studies have assessed thepotential of biomass based power generation for offsetting fossilfuel carbon emissions [7–9]. Accordingly, there is a significantknowledge base established, however, the information has not yetbeen collected for a full sustainability assessment. This study willreview the information from the previous studies to assess theenvironmental, social, engineering and financial sustainability ofbiomass use in electricity generation.

2. Technologies

There are three primary technology categories used for thecombustion based conversion of biomass into electricity. Eachcategory has undergone significant development and therefore hasmany different methods available.

2.1. Pyrolysis

Pyrolysis is the thermal destruction of biomass in an anaerobicenvironment, without the addition of steam or air to produce gasesand condensable vapours [10]. Combustion of these gases occurs ina gas turbine, typically combined cycle [11].

2.2. Gasification

In gasification, biomass is partly oxidised by controlling oxygenby the addition of steam to produce combustible gases, which havea high calorific value [10]. Product gases are fed into a combinedcycle gas turbine power plant [12].

2.3. Direct combustion

Direct combustion is the complete oxidation of biomass inexcess air, to produce carbon dioxide and water. Hot flue gases areused to heat process water to steam, which drives a turbine,typically via a Rankine cycle [10].

Direct combustion is the oldest and simplest, but most inefficienttechnology. Gasification and pyrolysis have higher efficiencies, butrequire significantly more process control and investment.

3. Biomass types

There are many biomass types available for the production ofelectricity, as shown in Table 1. This discussion is limited tobiomass residues and dedicated energy crops. Residues are wasteproducts after a higher value product has been obtained. In thecase of bagasse, it is the sugar cane residue once sugar andmolasses have been extracted. It can also be the tops and leaves ofthe sugar cane. Dedicated energy crops are grown exclusively forthe purpose of energy production.

3.1. Residues

3.1.1. Bagasse

Bagasse electricity generation is a proven process, taking thewaste products generated on-site and re-using them directly to

Table 1Biomass types.

Residues

Agricultural crop and process residues

Bagasse

Other

Forestry residues

Wood wastes

Dedicated Energy Crops

Food competitive

Short rotation croppice

Arid/unusable land

Mallee

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–1427 1421

power the process. There is typically some surplus generated thatis sold to the local grid. Waste heat after power generation istypically applied to sugar refining. In this way, costs andtransportation are minimal. The inherent linkage provides oneof the most sustainable methods of electricity generation available.However, the seasonality of sugar cane harvesting may limitapplicability of bagasse as a stand alone product. This limitationdoes not exist when used solely in conjunction with sugarproduction as no electricity is required when sugar cane isunavailable.

Sugar cane is produced in over 100 countries worldwide. Themain sugar producing countries are shown in Fig. 3 [13]. Brazil isby far the most dominant sugar producing country, accounting fornearly 35% of global production in 2007, followed by India at over22%. All other countries produce much less, but with manycountries such as China, Thailand and Australia still producingconsiderable amounts. In 2005, over 1015 tonnes of bagasse wereconsumed for power production [13].

3.1.2. Forest and non-bagasse agricultural residues

There are many benefits of using residues for energy produc-tion, including redirecting a waste product from landfill [14] andthey can be obtained at little or no cost. Available wastes includelarge amounts of leftover material, such as stalks, prunings, skins,shells and off-cuts from rice, grain, cotton, vegetables and fruit thatare not used as agricultural products. However, residues are a lowdensity and low value fuel where transportation costs per unit ofenergy are high [15]. Additionally, agricultural wastes have limitedquantities, they are location specific and not always of the idealquality for power generation [16].

3.2. Dedicated energy crops

For biomass to play a significant role in the world’s energyfuture, dedicated energy crops are essential. Short rotation energy

Fig. 3. Sugar cane production by country, 2007 (FAO, 2009) [13].

crops are gaining popularity internationally. Typical crops includepoplar, willow, eucalyptus and non-woody perennial grasses, suchas miscanthus. Crop rotation periods are usually 3–10 years forwoody crops, such as willow and poplar [17]. Important issues tobe addressed in dedicated energy crops are the depletion of soilnutrients, organic matter and moisture-holding capacity.

Ideal biomass characteristics are high yield, low input energy,low cost, composed with least amount of contaminants and withlow nutrient, water, pesticide and fertiliser requirements. Essentialbiomass properties are moisture content, calorific value, percent-age fixed carbon and volatiles, ash/residue content, alkali metalcontent, cellulose to lignin ratio and bulk density [18].

Crops grown solely for their energy content should give goodcrop yields. Switchgrass and wood from short rotation forestry,such as poplar and willow, are ideally suited as energy crops [19].Crops such as wheat and corn, require applications of fertiliser,take up prime agricultural land and give low crop yields, makingthem unsuitable energy crops [19].

One of the key concerns regarding bioenergy crops is the loss ofbiodiversity. This concern is due to monocultures being less stablethan forests and requiring increased energy inputs, such as pesticidesand fertilisers, to maintain productivity [20]. To mitigate this issue,IEA Bioenergy [15] suggests retaining patches/riparian corridors ofnatural vegetation as well as re-establishing native vegetation.

Biomass feedstock yields vary considerably, according toclimatic and soil conditions, agriculture and silviculture practices.The competitiveness of biomass is essentially dependent on thedevelopment of short rotation croppice and enhanced yieldswithout great cost increases [21]. Heller et al. [22] found thatwillow biomass crops are sustainable from an energy balanceperspective and contribute environmental benefits.

The Clean Energy Council (CEC) [23] found that, unlessstationary energy prices and CO2 taxes increase significantly,dedicated energy crops are unlikely to be economically viable. TheCEC [24] also state that dedicated energy crops only make sense inthe medium term for combating salinity, land erosion and loss ofbiodiversity. Dedicated energy crops are less viable than crops withmultiple economic benefits. Strezov et al. [25] and Hoogwijk et al.[26] believe that the production of energy crops on abandonedagricultural land and land at rest may allow for large amounts ofbiomass to be produced at reasonable cost. Koh and Hoi [27] claimthere is insufficient surplus agricultural land for dedicated energycrops to meet demand.

4. Sustainability assessment

To conduct this assessment, several key sustainability indica-tors were identified, the price to produce electricity, efficiency ofenergy conversion, total carbon dioxide emissions, availability,limitations, water use and social issues. All indicators are assessedover the entire life cycle on a per kilowatt hour basis.

Biomass conversion processes include thermal conversiontechnologies where the biomass energy is converted to electricityvia combustion. Power production from methane emitted due todecomposing landfill and sewage are also considered biomasscombustion based electricity.

Significant amounts of biomass are used in co-combustion withcoal, typically up to around 15% biomass. Overall, this is not arenewable process and the co-combustion with coal is excludedfrom discussion in this work.

The most common types of biomass used for electricityproduction and analysed in this work are agricultural residues,forest residues and dedicated energy crops. Other biomass sourcesinclude landfill gas, animal and human waste. However, as plantmaterial is the most common form of biomass used for electricitygeneration, the other sources will not be discussed in this work.

Table 2Literature prices for biomass power production.

Author/s Year c/kWh (US) Technology Size (MW) Fuel

Ganesh and Banerjee 2001 3.8–10.2 Pyrolysis 5 Energy crops

Bridgwater et al. 2002 9.4 Pyrolysis 20 Wood chip

Elliot 1993 7.8 Gasifier 25 Low cost plantation

Bridgwater 1995 6 Gasifier 60 Wood

Craig and Mann 1996 6.5–8.2 Gasifier 56–132 Wood

Faaij et al. 1997 �7.5 to +9.6 Gasifier 30 Wastes and residues

Faaij et al. 1998 7.7 Gasifier 30 Willow

McKendry 2002 16.4 Gasifier 7.5 Energy crops

Hamelinck et al. 2005 4.2 Gasifier 300 Wood

Gan and Smith 2006 5 Gasifier 10+ Poplar

Braunbeck et al. 1999 4.5–7.1 Combustion Large Bagasse

van den Broek et al. 2000 4.9 Combustion 23.4 Bagasse

van den Broek et al. 2001 9.1 Combustion 24 Wood

van den Broek et al. 2002 7.5 Combustion 13.5 Energy crops

Fung et al. 2002 2.8–8.5 Combustion 10 Wood

Bain and Overend 2002 8–12 Combustion 20 Mixed

Gustavsson and Madlener 2003 4-6.5 Combustion 50–100 Logging residues

Gustavsson and Madlener 2003 3.9–6.9 Combustion 50–100 Logging residues

Kumar et al. 2003 6.3 Combustion 137 Forest harvest residues

Kumar et al. 2003 5 Combustion 450 Agricultural residues

Kumar et al. 2003 4.7 Combustion 450+ Whole forest harvesting

Bakos et al. 2008 12.7 Combustion 2+ Agricultural residues

Alonso-Pippo et al. 2008 6 Combustion 600 Bagasse

Blanco and Azqueta 2008 9.3–12.4 Combustion 25 Straw

Kumar et al. 2008 5.4–5.9 Combustion 240 Trees killed by pine beetle

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–14271422

The sustainability assessed here considers the entire life cycle,from biomass growth, collection and transportation to powerproduction and waste disposal. Power plant life times are assumedas 20 years.

4.1. Price

4.1.1. Total cost of electricity production

Variability in feed materials and processing technologies resultsin large biomass price variations, shown in Table 2 [5,8,11,28–45].Lowest values are seen for waste materials with little or no cost andminimal transportation requirements. Due to the low energydensity (energy yield per hectare) of most biomass and high costsof transport from gathering site, any transportation significantlyaffects the resulting feedstock cost [41]. A high biomass density istherefore essential to profitability. Biomass quantities and costsvary with fluctuating harvests, increased utilisation by competi-tors and transportation [47].

There is wide debate on the cost effectiveness and majorinfluences on price. Bridgwater [33] claims that the economics liein low cost waste or fiscal incentives regardless of scale, as opposedto Siewert et al. [48] who state that 50–100 MW plants offergreater economy than smaller plants, without the need forfinancial support. According to Dornburg and Faaij [49] woodwaste needs to be provided at zero cost to make profits fromelectricity generation possible. This agrees with the findings ofGanesh and Banjeree [11] confirming the largest influence on priceis the fuel cost. It is in contrast with IEA Bioenergy [15], who foundthat energy crops are typically low value products and thatprofitability comes from low production costs.

The negative cost value given by Faaij et al. [38] is for instanceswhere current waste disposal costs can serve as income for thefacility. With prices ranging from �7.5 to 16.4 c/kWh and anaverage price of 6.9 c/kWh, biomass power production is not costeffective at present, where fossil fuel technologies are available foran average of 4.2–4.8 c/kWh. However, according to Saez et al.[50], when externalities, such as human health, soil erosion, etc.are included, the total price of biomass is cheaper than coal. Hatjeand Ruhl [51] state that biomass is the most profitable renewableenergy source after hydropower, with respect to total energy and

carbon reduction costs. Comparing to the median electricity costsof the remaining renewable electricity technologies shown byEvans et al. [52], biomass is cheaper than photovoltaics (24 c/kWh), approximately equal with geothermal (6.8 c/kWh) but moreexpensive than wind (6.6 c/kWh) and hydro (5.1 c/kWh).

4.1.2. Investment costs and technology choice

Investment costs for biomass to energy conversion exceed otherthermal technologies by a factor of 3–4 due to higher processingvolumes and increased handling requirements. The capitalintensive nature of biomass technology can deter investment.Also, financing biomass plant construction can be complicatedbecause many conversion technologies are still in pilot scale [23].

When selecting between different technologies, combustionbased technologies are more profitable over their life cycle thangasification and pyrolysis, despite higher operating costs [53].Capital costs for direct combustion are around $1.9–2.9/kW. Forpyrolysis, costs are much higher at $3.5–4.5/kW, making it one ofthe most capital intensive electricity generation technologies [54],comparable with nuclear.

4.1.3. Process versus feedstock price

Blanco and Azqueta [31] found that the fuel cost comprisedbetween 56 and 75% of the total price when using straw in Spain.

Gan and Smith [8] found that the non-fuel costs of capital,maintenance and operation of biomass fired electricity generationwas nearly the same as the total electricity production cost of coalsystems. They also found that the fuel accounted for approximately50% of the total electricity cost for biomass gasification systems.Biomass procured from logging residues was more cost effectivethan energy plantations.

Stucley et al. [55] showed that fuel procurement costs canaccount for some 50–60% of the total bioelectricity productioncosts. The total delivered fuel cost is typically 25% biomassproduction and 25% transportation to the power plant, withharvesting accounting for the remaining 50% of the total deliveredfuel cost. Accordingly, reductions in fuel costs will improve overallpower supply prices.

McIlveen-Wright et al. [56] found that dedicated forestry cropplants handling over 500 dry tonnes per day could be economically

Table 3The efficiency of energy conversion from biomass to electricity.

Author/s Year % Efficiency Comment

Craig and Mann 1996 35.4–39.7 Gasifier

Gustavsson 1997 36 Combustion

Faaij et al. 1997 35.4–40.3 Gasifier

Bain et al. 1998 35

Stahl and Neergaard 1998 32 Gasifier

Chum and Overend 2001 17.2 Gasifier

Berndes 2001 20–25 Combustion

Ganesh and Banerjee 2001 26 Combustion

Ganesh and Banerjee 2001 40 Combustion

Ganesh and Banerjee 2001 28 Gasifier

Ganesh and Banerjee 2001 31 Pyro lysis

Bain and Overend 2002 20

IEA Bioenergy 2002 25 5–10 MW

McKendry 2002 30 Gasifier

Gustavsson and

Madlener

2003 30 Combustion

Gustavsson and Madlener 2003 43 Combustion

la Cour Jansen 2003 24

Yoshida et al. 2003 19–26 Combustion

Yoshida et al. 2003 16–30 Gasifier

Benetto et al. 2004 22

Corti and Lombardi 2004 35 Gasifier

Siewert et al. 2004 35 Foster Wheeler

high efficiency

Franco and Giannini 2005 15–30 Small to lg, depends

steam temp

Ahrenfeldt et al. 2006 25 Wood to electricity

WEC 2007 20

Table 4Full life cycle carbon dioxide emissions from biomass power production.

Year Author/s Year gCO2/kWh Comment

1998 Faaij et al. 1998 24

1999 Norton 1999 30–40

2003 Gustavsson and

Madlener

2003 48 steam turbine

2003 Gustavsson and

Madlener

2003 37 CC

2007 Chatzimouratidis

and Pilavachi

2007 58 eq

2007 Styles and Jones 2007 131 eq miscanthus

2007 Styles and Jones 2007 132 eq SRC willow

*eq denotes CO2 equivalent in these values.

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–1427 1423

viable. Plants handling over 1000 dry tonnes per day would becompetitive with coal if there is enough wood available. Accordingto other authors, such as Koh and Hoi [27], it is unlikely that therewould be sufficient wood available to meet this target sustainably.

4.2. Efficiency

Efficiencies of energy conversion from biomass vary widelyacross different technologies. This is an area under intensedevelopment, with many new, highly efficient technologiesemerging.

Table 3 [10,11,15,29,35,38,40,48,54,57–67] summarises efficien-cies found in literature. Combined cycle gasification processes showthe greatest efficiencies at up to 43% [40]. The average efficiency of alltechnologies is 27%. There are diminishing returns from efficiencyimprovements, as small improvements at low efficiencies signifi-cantly affect profit margins, while at high efficiencies largeimprovements are necessary for the same gain [36].

As process efficiencies improve, greater attention will need tobe given to efficiencies of cultivation (if applicable), collection andtransportation of fuels. Improvements in this area will allow forsignificant price reductions.

4.3. Greenhouse gas emissions

Power production from biomass is often said to be carbonneutral. In some instances it is claimed that carbon sequestrationto plant and soil, along with non-invasive farming methods makebiomass electricity carbon negative, that is, less carbon is emittedthan is removed from the atmosphere overall [15]. Many authorsassert carbon neutrality, with emissions from combustionbalanced by carbon capture of the next crop [50,68–71]. Thereis inevitably some fossil fuel usage not balanced by this equation,resulting from fertiliser, cultivation, collection and transportation.According to some authors, harvest methods that removevegetation at or above soil level, leaving roots in the soil, leavesufficient carbon to balance all other emissions and maintain

carbon neutrality [50,69–70]. Mann and Spath [72] claim netcarbon negativity, since the combustion of biomass avoidsanaerobic decomposition that results in methane emissions.

In most cases, authors find that electricity generation frombiomass produces low net carbon emissions, mostly in the form ofcarbon dioxide, as shown in Table 4 [37,40,73–75]. Othergreenhouse gases, such as methane and nitrous oxide are emittedin smaller amounts (2% or less of total emissions [76]). Whereemissions include methane and nitrous oxide, figures are reportedas carbon dioxide equivalent, or CO2eq. The average carbonemission in Table 4 is 62.5 gCO2/kWh. The highest emission,132 gCO2eq/kWh [75] is less than one third of the lowest naturalgas and one fifth of the lowest coal fired power station emissionsproven at present.

Wihersaari [76] calculated the minimum greenhouse gasreduction when substituting biomass in the place of fossil fuelsat 74%, up to a maximum of 98%.

The calculation of carbon emissions can be complex,particularly when land clearing and soil carbon balances areincluded. If not produced sustainably, biomass generatedelectricity can actually emit more carbon dioxide per kilowatthour than fossil fuels [15]. Biomass impacts are negative whennative vegetation is removed for establishing an energy cropplantation. The establishment of such biomass crops is notrenewable as its use as a fuel results in significant net carbondioxide emissions [77]. For this reason, carbon emissions mustinclude land clearing, deforestation and soil emissions [78].Changing land use patterns can also affect greenhouse gasemissions. Highest emissions are seen where grassland andbroadleaved forests are converted to arable cropland [79].

In dedicated energy crop cultivation, crops that grow with theleast maintenance requirements, in particular little or no fertiliser,and highest energy densities, give the lowest emissions. This is ofparticular importance as nitrous oxide has a much higher globalwarming potential (GWP) at 298 than carbon dioxide, with a GWPof 1 or methane with a GWP of 25 [80]. Also an importantconsideration in dedicated energy crops is optimisation ofharvesting time. Van Belle [81] showed that an increase in thediameter of wood residues being chipped from 4 to 16 cm reducedthe carbon emissions per megawatt hour by a factor of seven.

The consideration of carbon storage in soils is also essential.There is over 1200 Gt of carbon stored in the world’s soils, incomparison to the 550 Gt carbon stored above ground (mostly intrees) [78]. Modern farming methods rely on carbon sequestrationto soil in their carbon balance and take measures to prevent theloss of this carbon, such as using no-dig cultivation.

According to Tampier et al. [19], crop yield is the largestinfluence on greenhouse gas emissions. Higher crop yields givelarger carbon savings due to the carbon in the crops. Transporta-tion does not add greatly to carbon emissions. Process emissions,while significant, are easily balanced by the fossil fuel displace-ment through biomass use. Dornberg et al. [7] also found that

Table 5Emissions from alternate fuels, sources and technologies

(Galbraith et al., 2006) [82].

Fuel CO2eq/kJ

UK electricity grid 1996 160

Straw combustion 63

SRC woodchip combustion 21

FR woodchip combustion 19

SRC woodchip pyrolysis 14

FR woodchip pyrolysis 12

SRC woodchip gasification 9

FR woodchip gasification 8

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–14271424

increased crop yields reduce greenhouse gas emissions. In theirstudy using different crops, the most greenhouse gas friendly cropwas found to be hemp grown in the Netherlands.

Technology choice impacts emissions, with pyrolysis andgasification showing significantly lower emissions than directcombustion and gasification slightly lower emissions thanpyrolysis [82]. This is highlighted in Table 5 [82], also showingthe difference in emissions from alternative fuel sources. Strawshows the highest emissions, while emissions from forest residues(FR) are always lower than emissions from dedicated short rotationenergy crops (SRC). Although emissions from straw combustionare much higher than for other bio fuels, they are still less than 40%of the standard UK grid emissions for 1996.

4.4. Water use

Horticulture has significant water requirements [20] therefore,biomass will have a higher overall water use than coal. There is alsohigh levels of water pollution from the use of pesticides andfertilisers [20]. Significant amounts of water are used during thecultivation, harvesting, transportation and processing of biomass.Berndes [59] gives a net water use on lignocellulosic bioenergycrops of 50 Mg/GJ of energy in the crops.

Because the technology for processing is essentially the same,cooling water for biomass based power plant operation require-ments is similar to coal based power plants at 78 kg water/kWh ofelectricity [10].

4.5. Availability

The sustainability of biomass resources is dependent upon therate of regeneration versus the rate of consumption, including non-energy demands [14].

The availability of feedstock is problematic for large scalegeneration. Crops are cheaper when waste products are used,however, high demands on wastes will outstrip supply, increaseprices and traditional waste streams then become primaryproducts. Due to resource constraints, many biomass plantsoperate for limited time periods, while feedstock is available. Thisseverely limits the possible penetration of the biomass technology.For example, Matsumura et al. [83] explored the possibility ofusing Japan’s main agricultural waste, rice straw and residue, toproduce electricity and found that supplies would only besufficient to operate a plant for 2 months per year.

Where dedicated energy products are grown, they competewith agriculture for space. Continual population growth rates arealready causing food supply pressures and increasing starvationrates, making this competition highly undesirable.

Novel ideas for feedstocks are continually under development.For example, the Northern Territory government is helping toestablish a pilot plant producing electricity from the environmen-tally damaging mimosa weed (mimosa pigra). The perceivedbenefits of this project are the control and eventual eradication ofmimosa as well as remote area access to electricity [84].

It has been estimated by IEA Bioenergy [85] that there is aglobal potential for electricity production from biomass as highas 200 EJ/year. Kaygusuz [86] gave an estimated potential of270 EJ on a sustainabile basis, significantly higher than thesustainable potential of 100 EJ/year given by Parikka [87]. Itmust be noted that even the lowest of these values, 100 EJ/year,still represents 30% of the global total energy consumption for2004. There is significant room for increased utilisation of thisresource. The CEC [23] calculated the long-term potential ofbagasse at 7800 GW/year.

It is the opinion of the CEC [23] that biomass in stationary energywill almost always be from recovered wastes/by-products of highervalue processes. Supply will be subject to the long-term viability ofthe primary product. Australia has a long-term potential of 50 TWh/year from agricultural wastes and 5 TWh/year from wood relatedwastes [23]. The long-term potential from grain crop residues,primarily cotton and wheat, is 47 TWh/year [88].

Fertilisation impacts are a function of harvest intensity andshort rotation periods. Ash fertilisation may alleviate nutrientlosses. Protecting organic layers in soil from disturbance andcompaction helps to reduce runoff that causes stream andwaterbody contamination by soil and silt [15]. Poplar shortrotation forestry with a 2-year cycle yielded 16 dry Mg/ha/yearwith CO2 emissions 7330 kg/ha/year, mostly from diesel fuelledmachines. Fertilisers cause ammonia, methane and nitrous oxidesemissions and groundwater pollution by acids and nitrates [89].Toonen [90] studied miscanthus and received yields of over 20 t/hawith a net energy of 17 GJ/tonne produced. They found a longharvest window and low input of fertilisers and pesticides.

4.6. Limitations

Biomass use is resource and land constrained. The mostproductive crop land is agricultural pasture, otherwise used toproduce food. In areas where soils are less ideal, crop yields arelower, sometimes to the point where the energy density is too lowto be economical.

Crops must also be able to grow with minimal maintenance,including watering, fertiliser, pest and disease control. Highmaintenance requirements reduce the environmental benefitsand increase carbon emissions and costs.

Abbasi and Abbasi [20] found that forest products have a highereconomic value per kJ in their original form than when convertedto heat or gaseous energy.

The highly variable nature of the biomass feedstocks causescomplications. A product such as coal is carefully monitored andmaintained at steady calorific and ash levels, allowing for steadyprocess control and minimising fouling. Biomass does not allowthe same control, even when using the same crops significantvariations can be seen. The combustion of biomass also causes highlevels of boiler fouling and corrosion [33].

Jungfeng and Runqing [91] concluded that biomass supply byenergy crop cultivation cannot match full biomass demand, even ifall feasible lands were developed.

4.7. Land use

Land used for biomass growth will often compete with foodcrops, forest and urbanisation, however, in some situations,biomass growth can be used to rehabilitate degraded ormarginal soil. For example, the mallee plantations in WesternAustralia are successfully helping to resolve salinity problemswhere other plants could not survive. A pilot plant is developingideal conditions to produce electricity from this tree [92,93].Where environmental benefits are shared between several areas,economics should improve with the respect to electricity price

A. Evans et al. / Renewable and Sustainable Energy Reviews 14 (2010) 1419–1427 1425

allocation. In the instance of Mallee eucalypts, the cost ofenvironmental rehabilitation of the site should be accountedseparately to the electricity cost. The San Antonio sugar millestablished eucalyptus plantations on degraded soils and onsoils that are uneconomic to cultivate common agriculturalcrops [94].

Fthenakis and Kim [95] compared the land occupation ofdifferent electricity generating technologies and found electricityproduction from willow has significantly the highest landtransformation of any technology, at over 12500 m2/GWh, whileall other technologies were below 4500 m2/GWh.

The allocation of valuable agricultural land and the destructionof natural forestry for energy crops growth is unsustainable. Thesesituations should always be avoided and other alternatives sought.

4.8. Social impacts

There is a wide range of social impacts arising from theproduction of electricity from biomass. The extent of the impactwill depend on the type of crop, how and why it was cultivated, thetechnology used to produce the electricity and how that electricityis distributed.

Food competition is ultimately the key social issue to beaddressed. In many cases, energy crops compete with food cropsfor valuable agricultural land. To avoid this competition, energycrops need to be grown only on agricultural land not used for foodcrops.

Along with agricultural land, forests are an essential site forbiomass crop growth. The removal of wood waste from forests canbe partially compensated by returning wood ash, rich in mineralnutrients and counteracting acidification, however, nitrogen andorganic matter are lost, the effects of stump harvesting and loss ofbiodiversity are not balanced [96]. Møller [96] also recommendsthat dead wood of a large size should be left as habitat for wildlife.The loss of habitat and biodiversity are key influences on the lack ofpublic acceptance and support for the use of native forest residues,which are the main available biomass resource [97]. There is thegeneral public perception that biomass power is not environmen-tally friendly [16]. Even scientific authors, such as Miranda andHale [98], conclude that natural gas shows more favourablecombined economic and environmental costs than biomasstechnology. IEA Bioenergy [15] conclude that the increasedproductivity of short rotation crops over forestry creates a smallerphysical footprint, which is an important consideration.

Direct labour inputs for wood biomass are two to three timesgreater per unit energy than for coal [20]. There is also anincreased labour requirement for construction, operation andmaintenance. The employment generated by the production ofelectricity from fuel oil is 15 person.yr/MW.yr, compared to32 person.yr/MW.yr for biomass [46]. More occupational inju-ries and illnesses are associated with biomass in agriculture andforestry than with underground coal mining, oil or gasextraction. Agriculture has 25% more injuries per man day thanall other private industries [20]. If the safety of the agriculturalsector could be improved, this would become a lucrativeemployment opportunity.

Taking waste wood from poor communities may remove self-sufficiency in areas where wood fuel is their only source ofheating. Efforts must be made when establishing biomasssources to ensure competing users are not disadvantaged bybiomass removal.

Comparing biomass with fuel oil, emissions of CO2 and SO2

equivalents are, respectively, 67 and 18 times lower [46]. Withadequate flue gas cleaning and particulate removal, biomass powergeneration could be a much cleaner option than alternate fossilfuel technologies in terms of all pollutants.

5. Conclusions

The generation of electricity from biomass faces variousenvironmental, technological and social challenges. Electricityprice, efficiencies, greenhouse gas emissions, availability andlimitations for biomass produced electricity are currently favour-able, when compared with the other energy generation options,however, significant attention must be given to reducing the landand water use and, the social impacts of biomass power generationbefore sustainability can be achieved.

Sustainable power production can be achieved by growinghardy crops on marginal or otherwise unusable land, such as seenin Western Australia with the mallee tree or Nicaragua witheucalyptus. Integrated solutions like bagasse are also sustainableas the fuel is a waste product directly generated and reused on-site,reducing land use, water use and social impacts. The leastsustainable biomass generation example occurs when energycrops compete with food crops or when energy crops are grownusing high amounts of fertilisers.

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